The mole works for its living by digging a tunnel through the soil, which it then patrols, gratefully eating such grubs or worms as come up into its tunnel or inadvertently drop in through the roof. Mole-draining similarly consists of driving channels three or four inches in diameter about two feet below the surface. This is done by bullet-shaped lumps of metal fixed on stout, knife-edged vertical beams or coulters, the whole apparatus drawn slowly forward by a massive caterpillar tractor. These ‘crawler’ tractors make poor progress on roads, so that they are most useful to the farmer whose fields lie comfortably around the homestead. They are slow and look as if their weight would compact the soil more than the heaviest tractor, but the reverse is true, because the weight is distributed along the whole length of the two caterpillars. Moreover, on a wet soil, you will see even a four-wheel-drive tractor with its wheels spinning, smearing the soil into a useless paste. This cannot happen with a caterpillar, whose ponderous strength – if the farmer has kept a machine of this sort – will without skidding pull out tractor, cultivator and all.